2 Watt Audio Amplifier
Notes:
This was one of the earliest circuits that I ever
designed and built, in Spring 1982. At that time I had only an analogue meter
and a calculator to work with. Although far from perfect, this amplifier does
have a wide frequency response, low distortion, and is capable of driving an 8
ohm speaker to output levels of around 5 watts with slightly higher distortion.
Any power supply in the range 12 to 18 Volts DC may be used.
Circuit Description
The amplifier operates in
Class AB mode; the single 470R preset resistor controls the quiescent current
flowing through the BD139/140 complimentary output transistors. Adjustment here,
is a trade-off between low distortion and low quiescent current. Typically,
under quiescent conditions, standby current may be 15 mA rising to 150 mA with a
50 mV input signal. A simulated frequency response is shown
below:
The circuit is DC biased so that the emitters of the BD139 and BD140 are
at approximately half supply voltage, to allow for a maximum output voltage
swing. All four transistors are direct coupled which ensures:-
(i) A good low
frequency response
(ii) Temperature and bias change stability.
The
BC109C and 2N3906 operate in common emitter. This alone will provide a very high
open loop gain. The output BD139/140 pair operate in emitter follower, meeting
the requirements to drive low impedance speakers. Overall gain is provided by
the ratio of the 22k and 1k resistor. A heat sink on the BD139/140 pair is
recommended but not essential, though the transistors will run "hot" to the
touch.
Original March 1999
Last Revision 13-8-01 by
ANC